The invention relates to a method of manufacturing a hybrid integrated circuit comprising a semiconductor element and a piezoelectric filter which are provided next to each other on a carrier substrate, with the semiconductor element comprising semiconductor zones provided in a layer of silicon, and the piezoelectric filter comprising an acoustic resonator formed on an acoustic reflector layer, said acoustic resonator including a layer of a piezoelectric material, a first electrode situated between this layer and the acoustic reflector layer, and, on the other side of the layer of piezoelectric material, a second electrode situated opposite the first electrode.
The semiconductor element may be a single transistor but also an integrated circuit formed in a layer of semiconductor material and comprising a large number of transistors to which, if necessary, passive components may be added. Piezoelectric filters, also referred to as xe2x80x9cThin Film Acoustic Wave Resonatorsxe2x80x9d can be manufactured so as to have resonant frequencies in the range between 500 MHz and 5 GHz, quality factors Q above 1,000 and small dimensions, for example a length and a width of 200 xcexcm. The use of such filters enables hybrid integrated circuits, such as selective amplifiers, to be formed on the carrier substrate, which are particularly suitable for use in equipment for personal wireless communication, such as GSM telephony, with which signals of said frequencies are processed.
In practice, the layer of piezoelectric material may be, for example, a layer of aluminium nitride AlN or zinc oxide ZnO. These layers are applied in a thickness equal to half the wavelength with which acoustic waves of said frequencies propagate in these materials. The acoustic reflector layer on which the resonator is situated is generally composed in practice of several sub-layers of alternately a high and a low acoustic impedance. Use is customarily made of sub-layers of, for example, tungsten having a relatively high acoustic impedance of approximately 100 Gg/m2s and, for example, silicon oxide or a synthetic resin having a relatively low acoustic impedance of, respectively, approximately 13 Gg/m2s and approximately 2 Gg/m2s. These layers are applied in a thickness which is equal to a quarter of the wavelength with which acoustic waves of said frequencies propagate in these materials. For frequencies in said frequency range, both the piezoelectric layers and the reflector layers have thicknesses in the range from 1 to 3 xcexcm.
U.S. Pat. No. 3,414,832 discloses a method of the type mentioned in the opening paragraph, where, in a first example, there is started from a carrier substrate of silicon. In this substrate, the semiconductor element, i.e. a bipolar transistor, is formed. Subsequently, the piezoelectric filter is provided next to the semiconductor element. For this purpose, first an acoustic reflector layer is locally formed on the substrate, whereafter the acoustic resonator is provided on this layer. Finally, a metallization is formed which connects the semiconductor element with the filter. In a second example, there is started from an insulating, ceramic substrate on which first a semiconductor crystal including the semiconductor element is provided. From this point, the method is carried out in the same way as described with respect to the first example.
In practice, it has been found to be difficult to provide the acoustic reflector layer next to the semiconductor element. For this purpose, such a layer must be deposited both on and next to the semiconductor element, whereafter it must be patterned by means of an etch treatment, as a result of which the layer is removed again from the semiconductor element. Deposited layers may exhibit local differences in thickness, and etch processes may locally take place at different etch rates. Consequently, in order to make sure that the layer is entirely removed from the semiconductor element, the etch treatment is carried out for a period of time exceeding the time strictly necessary to remove the layer. In practice, an xe2x80x9coveretch timexe2x80x9d of approximately 20% is normal. As the acoustic reflector layer is thick in comparison with layers used in semiconductor elements, it is possible that layers in the semiconductor element are entirely etched away during the xe2x80x9coveretchxe2x80x9d timexe2x80x9d. This may result in such damage to the semiconductor element that it can no longer be used.
It is an object of the invention to overcome the above-mentioned drawback. To achieve this, the method in accordance with the invention is characterized in that the semiconductor element is formed at a first side of an auxiliary slice of silicon, after which the layer of piezoelectric material supporting the first electrode is provided, at the same first side, on the auxiliary slice, whereafter the structure thus formed is provided over its free surface area with an acoustic reflector layer and, subsequently, attached to the carrier substrate with this layer, whereafter silicon is removed from the second side of the auxiliary slice at the location of the acoustic resonator.
After the formation of the semiconductor element, a layer of a piezoelectric material is deposited on the auxiliary slice of silicon and subsequently etched in accordance with a pattern. This layer is thin as compared to the acoustic reflector layer. The thickness of the layer of piezoelectric material practically ranges between 1 and 3 xcexcm. Such a thin layer can be readily etched in accordance with a pattern without layers of the semiconductor element being attacked to such an extent by an xe2x80x9coveretch timexe2x80x9d that the semiconductor element becomes useless. The acoustic reflector layer which is applied to the semiconductor element and to the acoustic resonator is not etched in accordance with a pattern. The hybrid integrated circuit can thus be formed on the carrier substrate without the acoustic reflector layer having to be etched in accordance with a pattern.
Preferably, the semiconductor zones are formed in a top layer of a slice of silicon, which slice is provided with a silicon oxide layer situated on the top layer, after which the layer of piezoelectric material supporting the first electrode is formed on this layer of silicon oxide, whereafter the free surface of the structure thus formed is provided with an acoustic reflector layer, and the structure is subsequently attached to the carrier substrate with said layer, after which the surface of the second side of the auxiliary slice is subjected to a silicon-removal process which stops just short of the top layer, and subsequently, at the location of the acoustic resonator, silicon is removed down to the layer of silicon oxide. The first silicon-removal step, which takes place throughout the surface, may be carried out by means of a customary mechanical-chemical polishing treatment. In the second step, wherein the layer of silicon oxide is exposed at the location of the acoustic resonator, the silicon oxide layer may serve as an etch-stop layer. In an etch bath containing potassium hydroxide, silicon can be etched very selectively with respect to silicon oxide. During this etching treatment, the auxiliary slice must be masked at the location of the semiconductor element to preclude silicon from being removed at said location.
The silicon can be even more readily removed from the second side of the auxiliary slice if the semiconductor zones are formed in a top layer of a silicon slice provided with a silicon oxide layer buried under the top layer, which top layer is removed next to the semiconductor element, whereafter the layer of piezoelectric material supporting the first electrode is formed on the silicon oxide layer thus exposed, after which the free surface of the structure thus formed is provided with an acoustic reflector layer, whereafter the structure is attached to the carrier substrate with this layer, after which silicon is removed from the surface of the second side of the auxiliary slice down to the buried layer of silicon oxide. The slice used is an SOI (Silicon-On-Insulator) slice. Also in this case, a customary mechanical-chemical polishing treatment can be carried out as a first step in the removal process. In a subsequent, maskless step, silicon can be etched away until the buried layer of silicon oxide is exposed. Of an approximately 600 xcexcm thick slice, for example, approximately 500 xcexcm can be removed by the polishing treatment and the remainder by etching.
After the formation of the piezoelectric layer of the resonator on the auxiliary slice, the first electrode is formed thereon. The second electrode opposite the first electrode is provided on the resonator in a simple manner if, prior to the formation of the acoustic resonator on the silicon oxide layer situated at the first side of the auxiliary slice, the second electrode is provided, at the location of the acoustic resonator, after which the layer of piezoelectric material carrying the first electrode is formed on this second electrode.
A drawback of this method resides in that the layer of piezoelectric material of the resonator must be formed on said second electrode. To preclude damage to this electrode, the layer of piezoelectric material must be deposited at a comparatively low temperature. If the second electrode is formed, for example, in a layer of aluminium or tungsten, then the auxiliary slice must not be heated to temperatures above 350xc2x0 C. during the deposition of the layer of piezoelectric material. To obtain a layer of piezoelectric material comprising equally oriented crystals, it may be desirable to deposit the layer at a higher temperature. This is possible if the layer of piezoelectric material is formed directly on the silicon oxide layer situated at the first side of the auxiliary slice, after which the first electrode is provided, the free surface of the structure thus formed is provided with an acoustic reflector layer, and subsequently said structure is attached to the carrier substrate with this layer, after which the layer of silicon oxide is exposed from the second side of the auxiliary slice, and subsequently this layer is provided, at the location of the acoustic resonator, with a window wherein the second electrode is provided. The layer of piezoelectric material is thus provided prior to the formation of the second electrode. Damage to this electrode caused by heating during the deposition process is thus precluded.
The piezoelectric filter must be connected in the integrated circuit to the semiconductor element. If both the first and the second electrode are connected to the semiconductor element, then two metallization layers must be formed. That is to say, viewed from the auxiliary slice, one metallization layer under the resonator and one metallization layer on top of the resonator. Since the resonator is comparatively thick, i.e. the thickness ranges from 1 to 3 xcexcm, providing the second metallization layer is not easy. To provide this layer, it must comprise conductor tracks which extend both on and next to the comparatively thick resonator, but also conductor tracks which extend over the edge of the resonator. Particularly the latter conductor tracks are difficult to provide as they must bridge a comparatively large difference in height. This problem is circumvented if the second electrode is embodied so as to be two sub-electrodes which are both situated opposite the first electrode, so that two series-connected resonators are formed between the two sub-electrodes. In this case, only a metallization has to be formed on and next to the resonator, and said step to bridge the difference in height does not have to be taken.
A simple construction is further obtained if the auxiliary substrate is attached to the carrier substrate by means of an adhesive layer which forms part of the acoustic reflector layer, in particular if also the first electrode of the acoustic resonator forms part of the acoustic reflector layer. In practice, this is possible because an adhesive layer has a comparatively low acoustic impedance and electrode materials have a much higher acoustic impedance. The thickness of the first electrode must be chosen so as to correspond to the desired resonant frequency of the filter; the adhesive layer may be much thicker on account of its very low acoustic impedance.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.